1. Field of the Invention
The present invention relates to an optical recording medium, more particularly an optical recording medium using an organic dye as a recording layer, and an optical recording-reproducing method using thereof.
2. Description of the Related Art
As an optical recording medium, an optical recording medium using an optical organic dye material in a recording layer is configured to have a recording layer whose principal ingredient is an organic dye, and also a reflection layer, a protection layer, and the like according to a necessity, on a transparent substrate such as a plastic, and information is recorded in high density by scanning laser light on a surface of this recording layer and by forming a pit only in a portion of the irradiated surface.
An organic dye recording medium which is represented by a CD-R (write-once type CD: Compact Disc, registered trademark) is widely accepted in the market as a backup of data, image, and the like and also as a music use since a structure is simple and it is possible to manufacture at a low cost. However, there is a desire for a recording medium having a further larger capacity along with a rapid progress of a computer in recent years, and a demand for the optical recording medium using the organic dye is shifting from the CD-R and the like to a DVD−R and DVD+R (right-once type DVD: Digital Versatile Disc, registered trademark).
Recording of information onto the optical recording medium using the organic dye is performed in heat mode. When the laser light is irradiated to the recording layer, the organic dye of the irradiated portion is resolved by absorbing the laser light to generate heat. Since the portion where the organic dye is resolved has a different reflectivity from an unresolved portion, a pit is to be formed inside the recording layer. Accordingly, a mechanism to absorb the heat becomes important in the optical recording medium using the organic dye.
An amount of heat necessary for resolving the organic dye inside the recording layer is determined by a light absorption spectrum. Generally, when the absorption of light becomes large in a certain wavelength, a recording sensitivity becomes high although a reflectivity is decreased. On the contrary, when the absorption becomes small, the reflectivity becomes high although the recording sensitivity is decreased.
Although a reflectivity characteristic and a recording sensitivity characteristic of the recording layer are important in the optical recording medium using the organic dye, those characteristics have a mutually close correlation as described hereinbefore and therefore give a big influence on a recording characteristic of the optical recording medium. In the DVD, the laser light of a shorter wavelength of 650 nm (DVD−R) or 655 nm (DVD+R) than the CD is used in order to increase a recording density, and the organic dye which has been previously used for the CD-R may not be used for the DVD−R and DVD+R due to a reason that the absorption is so large in this wavelength range as to shift greatly from the optimum recording sensitivity.
Moreover, even in the DVD−R and DVD+R, high speed recording is demanded similarly to the CD−R. In a recording material to perform the high speed recording, it is required that not only the reflectivity is high, but also the recording sensitivity is a high sensitivity, and a resolving speed of a dye due to heat is fast.
Various dye materials suitable for application to the DVD-R and the like have been proposed (for example, refer to patent references 1 and 2).
However, a recording-reproducing characteristic at the time of high speed recording is not sufficiently studied.
In addition, a deterioration of a signal characteristic at the time of low speed recording is observed when the material is optimized to the high speed recording by simply increasing the resolving speed of the dye, and also similarly an opposite case is that a signal characteristic at the time of high speed recording is deteriorated when the material is optimized to the low speed recording.
Furthermore, there is a possibility that a light stability characteristic deteriorates in the organic dye recording medium of high sensitivity.
Therefore, it is very difficult to satisfy simultaneously all of the recording characteristic and the light stability characteristic over low speed to high speed recording in one single organic dye recording medium.    [Patent Reference 1] Japanese patent application publication No. 2002-52829    [Patent Reference 2] Japanese patent application publication No. 2003-231359